Flat panel display devices such as liquid crystal displays which are distributed in the market at present have a structure as shown, for example, in FIG. 1. Specifically, a flat panel display device 5 has a backlight 2 on the back side of picture elements 4, and the backlight 2 has light sources 3 and a light diffusion plate 1 (hereinafter also referred to as a “light diffusion film” with respect to the present invention) for uniformly diffusing the light from the light sources 3 to the side of the picture elements 4. The mode of the backlight 2 is roughly divided into a direct type and an edge light type, and FIG. 1 shows a direct backlight. In the direct backlight, a plurality of light sources 3 are arranged directly behind the light diffusion plate 1, as shown in FIG. 1. Further, a reflector plate 6 is usually arranged on the opposite side of the light sources 3 from the light diffusion plate 1.
In the case of a direct backlight, since the light sources are arranged directly behind a light diffusion plate, the distance from the light sources to the light diffusion plate is generally short, and the light passage distance in the light diffusion plate is also short. Therefore, a light diffusion plate which can sufficiently diffuse the light from each light source is required so that the luminance unevenness resulting from using a plurality of light sources does not occur.
As a material for a light diffusion plate, there is known a light diffusion plate in which a light-diffusing agent is dispersed in a polycarbonate resin in order to increase light diffusibility (Patent Documents 1 and 2). For example, Patent Document 1 proposes a light diffusion film in which 1 to 20 parts by mass of particles having an average particle size of 1 to 25 μm are added to a polycarbonate resin to impart an uneven shape to the surface. Further, Patent Document 2 describes that light-diffusing agents such as inorganic particles and/or organic particles may be dispersed in a polycarbonate resin in the production of a light diffusion plate. However, conventional light diffusion plates (or light diffusion films) have had a problem that light diffusibility and light source shape-hiding properties are reduced as the thickness of the light diffusion plates is reduced.